Bearing protection for inverter-driven motor

ABSTRACT

A refrigerant motor/compressor employs both a serviceable shaft-grounding device and a ceramic coating to protect a rolling element bearing that could otherwise be damaged by high frequency induced common mode voltage and current originating from an inverter that includes a plurality of IGBTs (insulate gate bipolar transistors). The shaft-grounding device includes a stranded copper wire brush that rides against an axial end of the shaft and a high frequency stranded grounding wire that conducts the induced current away from the shaft. The shaft-grounding device is sized and positioned so that it can be momentarily removed for inspection without having to evacuate the refrigerant. The ceramic coating provides an electrical insulating surface on a bearing bracket and other parts that support the bearing. The coating comprises titanium dioxide and aluminum oxide to provide a surface that is sufficiently hard and tough to resist damage during assembly, thereby maintaining the coating&#39;s integrity.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The subject invention generally pertains to a hermetically sealed compressor with an inverter-driven motor and more specifically to a means for protecting the motor's bearings against certain induced voltages and currents.

2. Description of Related Art

For years, it has been known that shaft induced current, driven by shaft induced voltage, can damage motor bearings. In some cases, the energy generated by shaft induced current causes deterioration of the lubricant, which can ultimately damage the bearings. If the induced voltage is sufficiently high, electrical arching across the bearing can erode the bearing's surfaces directly.

Shaft induced voltage can come from different sources. It can be electrostatically generated within the motor, or the voltage can arise from imbalanced ampere-turns in the stator, or from stator or rotor asymmetries. In cases where the motor is driven by an inverter or variable speed drive, the induced voltage is known as common mode voltage, which can be caused by the switching frequency of the inverter's SCRs (silicone controlled rectifiers), BJTs (bipolar junction transistors), GTOs (gate turn off tyristors), or, more recently, IGBTs (insulate gate bipolar transistors). Additional background on shaft induced voltage and related information can be found in U.S. Pat. Nos. 5,313,129; 5,914,547; 6,030,128; 5,735,615; 5,139,425; 5,059,041; 4,109,978; 4,378,138; 4,220,879 and 6,555,943.

Although electrically grounding the shaft or electrically isolating the bearing can reduce the effects of shaft induced voltage, such measures are usually not necessary due to improvements in the design and manufacture of modem day motors and their variable speed drives. More recently, however, the SCRs, BJTs and GTOs of inverters have been replaced by much faster IGBTs. While SCR's, BJTs and GTOs operate at relatively low frequencies, IGBTs operate at switching frequencies of 2-4 kHz and higher. At these higher switching frequencies, IGBT's appear to generate common mode current in the range of 2 to 10 MHz, which can be very difficult to limit to a conductive path that bypasses the bearing.

The increasing popularity of IGBTs for variable speed drives has not only resurrected the problem of induced common mode voltage, it has raised the problem to a new level where conventional methods of correction no longer work. Consequently, a need exists for a way to protect the bearings of a motor driven by an inverter with IGBTs. A better method is particularly needed for hermetically sealed motors whose bearings are relatively inaccessible for repair.

SUMMARY OF THE INVENTION

It is an object of the invention to help prevent certain induced currents from damaging a rolling element bearing of a refrigerant compressor system driven by an inverter.

Another object of some embodiments is support a motor/compressor shaft with two different style bearings, a rolling element bearing and a journal bearing, where only the rolling element bearing needs a shaft-grounding device, which is generally accessible for servicing.

Another object of some embodiments is to use both shaft-grounding and electrical insulation to protect a bearing against induced common mode voltage originating from an inverter's IGBTs.

Another object of some embodiments is to provide a way of servicing a shaft-grounding device without adversely affecting the refrigerant charge of a hermetically sealed compressor system.

Another object of some embodiments is to contact the end of a shaft with a shaft-grounding device that applies an ideal magnitude of contact force.

Another object of some embodiments is to electrically insulate a bearing from an adjacent supporting member by coating the member with a ceramic layer that is harder than the outer periphery of the bearing and harder than the material of the supporting member, wherein the hardness of the coating is by virtue of the ceramic layer having certain proportions of titanium dioxide and aluminum oxide.

Another object of some embodiments is to provide an outboard with an adjacent labyrinth seal that inhibits excessive gas flow when the shaft-grounding device is momentarily removed.

Another object of some embodiments is to screw a shaft-grounding device into a threaded hole that is sufficiently small to minimize any gas exchange between the compressor and the atmosphere when the shaft-grounding device is temporarily removed.

Another object of some embodiments is to ground the end of a shaft using a stranded copper wire brush rather than using a carbon block, as the wire brush is more effective at conducting induced common mode current.

Another object of some embodiments is to align a shaft-grounding device with a rotational axis of a shaft to minimize wear between the shaft and the shaft-grounding device.

Another object of some embodiments is to provide the brush of a shaft-grounding device with some axial movement to ensure contact between the brush and the end of the shaft even after the brush experiences some wear.

Another object of some embodiments is to restrict relative rotation between a brush and an outer housing of a shaft-grounding device to prevent the shaft from rotating the brush and creating wear within the shaft-grounding device.

Another object of some embodiments is to use a stranded grounding wire to effectively convey high frequency common mode current from a shaft.

One or more of these and/or other objects of the invention are provided by a compressor system that employs both a serviceable shaft-grounding device and a ceramic coating to protect a rolling element bearing that could otherwise be damaged by high frequency induced common mode voltage and current originating from an inverter that includes a plurality of IGBTs.

The present invention provides a compressor system powered by an AC voltage supply for compressing a refrigerant. The system includes a compressor housing defining a suction inlet and a discharge outlet; a motor housing extending from the compressor housing; a bearing bracket extending from the motor housing; a bearing having an outer periphery supported by the bearing bracket; and a shaft supported by the bearing and being rotatable about a rotational axis. The shaft includes an outboard end and an inboard end. The bearing is closer to the outboard end than to the inboard end. The system also includes a compressor element driven by the shaft and rotatable relative to the compressor housing to force the refrigerant from the suction inlet to the discharge outlet. The compressor element is closer to the inboard end than to the outboard end. The system includes a minimally conductive coating disposed on the bearing bracket. The coating is between the bearing bracket and the bearing to provide electrical resistance therebetween, and the coating is harder than the bearing bracket and the outer periphery of the bearing.

The present invention also provides a compressor system powered by an AC voltage supply for compressing a refrigerant. The system includes a compressor housing defining a suction inlet and a discharge outlet; a motor housing attached to the compressor housing; a bearing bracket attached to the motor housing; a bearing supported by the bearing bracket; a shaft supported by the bearing and being rotatable about a rotational axis. The shaft includes an outboard end and an inboard end. The bearing is closer to the outboard end than to the inboard end. The system also includes a compressor element driven by the shaft and being rotatable to force the refrigerant from the suction inlet to the discharge outlet. The compressor element is closer to the inboard end than to the outboard end. The system further includes an endplate spaced apart from the shaft, spaced apart from the bearing, and electrically coupled to the compressor housing, a ceramic coating disposed on the bearing bracket and a shaft grounding device. The ceramic coating is between the bearing bracket and the bearing. The shaft-grounding device includes a wire brush, a brush housing, and a spring. The brush housing is attached to the endplate. The wire brush is movable along a longitudinal centerline of the brush housing. The spring urges the wire brush toward the outboard end of the shaft to create electrical continuity between the wire brush and the outboard end of the shaft. The wire brush is electrically coupled to the brush housing. The brush housing is electrically coupled to the endplate, thereby establishing electrical continuity between the compressor housing and the outboard end of the shaft while the ceramic coating provides electrical resistance directly between the bearing and the bearing bracket.

The present invention further provides a compressor system powered by an AC voltage supply for compressing a refrigerant. The system includes a compressor housing defining a suction inlet and a discharge outlet; a motor housing adjacent to the compressor housing; a bearing supported within the motor housing; and a shaft supported by the bearing and being rotatable about a rotational axis. The shaft includes an outboard end and an inboard end. The bearing is closer to the outboard end than to the inboard end. The system also includes a compressor element driven by the shaft and being rotatable to force the refrigerant from the suction inlet to the discharge outlet and a shaft-grounding device. The compressor element is closer to the inboard end than to the outboard end. The shaft grounding device includes a wire brush, a brush housing, a spring, and a grounding wire. The brush housing is electrically coupled to the motor housing. The grounding wire electrically couples the wire brush to the brush housing. The wire brush is movable along a substantially linear path. The spring is contained within the brush housing and urges the wire brush toward the outboard end of the shaft to create electrical continuity between the wire brush and the outboard end of the shaft.

The present invention additionally provides a compressor system. The system includes a motor housing having an interior containing a refrigerant and an exterior exposed to a surrounding atmosphere; a compressor housing hermetically sealed to the motor housing; an endplate extending from the motor housing; a refrigerant disposed within the compressor housing and the motor housing; a rolling element bearing inside the motor housing; a journal bearing inside at least one of the motor housing and the compressor housing; and a shaft having an inboard end, an outboard end, and an intermediate section therebetween. The rolling element bearing supports the outboard end, and the journal bearing supports the intermediate section. The system also includes a compressor element mounted to the inboard end of the shaft and being rotatable for compressing the refrigerant; and a shaft-grounding device extending into the opening of the endplate such that the shaft-grounding device is in electrical contact with the outboard end of the shaft and is exposed to the refrigerant and the surrounding atmosphere.

The present invention yet further provides a method of servicing a hermetically sealed compressor system that includes a motor housing exposed to a surrounding atmosphere, a shaft rotatable within the motor housing, a refrigerant disposed within the motor housing, and a shaft-grounding device that when properly installed is exposed to the refrigerant and the surrounding atmosphere and completes an electrical path between the shaft and the motor housing. The method includes the steps of adjusting the temperature of the refrigerant until the refrigerant in the motor housing is at a pressure substantially equal to that of the surrounding atmosphere; and removing the shaft-grounding device from within the motor housing while the pressure of the refrigerant is substantially equal to that of the surrounding atmosphere, thereby providing an opportunity to inspect the shaft-grounding device without having to evacuate the refrigerant from within the motor housing.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a compressor system connected to a schematically illustrated refrigerant circuit and inverter.

FIG. 2 is cross-sectional view of the compressor system of FIG. 1 showing one end of the compressor's motor.

FIG. 3 is a cross-sectional view of a shaft-grounding device attached to an endplate and engaging a shaft.

FIG. 4 is a cross-sectional view similar to FIG. 3 but showing the shaft-grounding device separated from the endplate and the shaft.

FIG. 5 is a cross-sectional view similar to FIG. 1 but showing how the shaft-grounding device can be temporarily removed for servicing.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 illustrates a hermetically sealed compressor system 10 comprising a compressor 12 and a motor 14. Compressor system 10 also includes a novel shaft-grounding device 16 for electrically grounding a shaft 18 and an electrically insulating material, such as a minimally conductive coating 20 (FIG. 2), for electrically isolating a conductive bearing 22 that supports shaft 18. An inverter 24 controls the speed of motor 12, which in turn drives compressor 14. The term, “hermetically sealed” refers to a non-sliding, substantially airtight transition 26 between a motor housing 28 of motor 12 and a compressor housing 30 of compressor 14 such that transition 26 does not provide a significant leak path for gas or refrigerant within system 10 to escape to an atmosphere 32 surrounding housings 28 and 30. Hermetic sealing of system 10 can be accomplished by a solid, airtight joint between housings 28 and 30, as shown, or by making housings 28 and 30 as a unitary piece.

In a currently preferred embodiment, compressor system 10 contains a refrigerant that compressor 14 forces from a suction inlet 34 to a discharge outlet 36, both of which are defined by compressor housing 30. Compressor system 10 can be used for powering a refrigerant circuit 38 comprising the basic components of a condenser 40, an expansion device 42, and an evaporator 44. In some embodiments of the invention, condenser 40 is water-cooled and evaporator 44 absorbs heat from a circulating water system 46. Pumps 48 and 50 can be used for controlling the flow rate of the water through condenser 40 and evaporator 44. The structure and function of refrigerant circuit 38 and its many variations are well known to those of ordinary skill in the art.

Although the actual structure of compressor system 10 may vary, the illustrated embodiment has shaft 18 supporting both a rotor 52 of motor 12 and at least one compressor element 54. The term, “compressor element” refers to any component that can be driven to compress a gas. Examples of compressor element 54 include, but are not limited to, a centrifugal impeller, an axial impeller, a multi-lobed screw compressor rotor, an involute scroll compressor rotor, a reciprocating piston, and the like. Although a single shaft 18 is shown supporting both rotor 52 and impeller 54, it is well within the scope of the invention to have rotor 52 and impeller 54 supported by two separate shafts that are coupled to each other by way of gears or some other appropriate coupling.

For this direct drive example, bearing 22 supports an outboard end 56 of shaft 18, and another bearing 58 closer to an inboard end 60 supports shaft 18 at an intermediate section 62 of shaft 18 such that shaft 18 supports compressor element 54 in a cantilevered manner. Bearing 22 is a rolling element duplex bearing for providing shaft 18 with both axial and radial support, and bearing 58 is preferably a journal bearing for pure radial support of the shaft intermediate section 62. Bearings 22, 58 are lubricated with a thin film of refrigerant/lubricant mixture. Rotor 52 is situated between bearings 22 and 58, and a stator 64 supported by motor housing 28 encircles the rotor.

Referring further to FIG. 2, motor housing 28 is shown supporting bearing 22 by way of a bearing bracket 66, a seal ring 68 and a clamp ring 70. Bearing bracket 66 is bolted to a cylindrical shell 72 of housing 28, and an inner bore of bracket 66 provides bearing 22 with radial support. In an axial direction, parallel to shaft 18, the outer races of bearing 22 are captured between clamp ring 70 and seal ring 68, which are both bolted to bearing bracket 66. A shoulder 74 and an internally threaded ring 76 axially clamp the inner races of bearing 22 to shaft 18.

To help prevent the lubricant for bearing 22 from freely draining into the main chamber of motor housing 28 and eventually becoming lost within refrigerant circuit 38, seal ring 68 includes a labyrinth seal 78 that is spaced just a slight radial distance away from shaft 18.

To drive compressor system 10 at various speeds, electrical cables 80 connect inverter 24 to the windings of stator 64 (stator 64 includes its windings and its core). One example of inverter 24 is a “LiquiFlo 2.0 AC Drive” manufactured by Reliance Electric, which is part of Rockwell Automation of Milwaukee, Wis. with further headquarters in Greenville, S.C. Inverter 24 includes a converter section 82 with a plurality of insulate gate bipolar transistors 84 for converting an incoming 3-phase AC supply voltage 86 to a DC voltage 88, and an inverter section 90 electrically coupled to converter section 82 and comprising a plurality of insulate gate bipolar transistors 92 for converting DC voltage 88 to a variable frequency 3-phase output voltage 94 that cables 80 feed to stator 64. In addition to their intended purpose, the plurality of insulate gate bipolar transistors 84 and 92 induce a potentially detrimental common mode current in shaft 18. The common mode current can exceed one megahertz (e.g., 2-3 MHz range) and has been observed to have a frequency as high as 10 MHz.

To inhibit bearing 22 from conveying the common mode current to an electrical ground 96, a non-conductive or minimally conductive coating 20 is disposed on several bearing-contact surfaces including a surface 98 of clamp ring 70, a surface 100 of seal ring 68, and the inner bore of bearing bracket 66. Coating 20 is preferably harder and less electrically conductive than the base material to which it is applied and harder and less conductive than an outer periphery 102 of bearing 22. Preferably, coating 20 is a ceramic coating but other insulative coatings are contemplated such as silicon oxides or metal oxides. For purposes of this application, a minimally conductive coating conducts at less than the dielectric strength of the elastohydrodynamic thickness of the film on the bearing. This will vary depending on the refrigerants and lubricants being used in a particular system.

In a currently preferred embodiment, coating 20 is a METCO 130 Alumima-Titania Composite Powder (METCO is a registered trademark of Sulzer Metco of Winterthur, Switzerland). The METCO coating is comprised of about 13% titanium dioxide and about 87% aluminum oxide. Coating 20 can be sprayed on selected surfaces of parts 66, 68 and 70 and subsequently machined or ground to size with a final layer thickness ranging from a few thousandth of an inch to 0.020-inches. The thickness of ceramic coating 20 has been exaggerated in the drawing figures so that the coating is clearly visible. With a hardness of 60 Rc, coating 20 is not readily scratched by bearing 22 or the other components of compressor system 10 during assembly.

Since coating 20 alone does not adequately solve the problem of induced common mode voltage, shaft-grounding device 16 is used for grounding shaft 18. In some cases, shaft 18 may include a bolt head 18′ or some other suitably conductive member that can be engaged by shaft-grounding device 16.

Referring further to FIG. 3, to successfully ground common mode voltage whose frequency is above 2-MHz, it has been found that shaft-grounding device 16 should have a stranded wire brush 104 made of copper and a stranded high-frequency grounding wire 106 that can effectively draw the current away from brush 104. Moreover, a spring 108 is needed to urge brush 104 against shaft 18 with an axial force 110 that is neither too great (to avoid excessive wear) or too light (to ensure continuous electrical contact). Force 110 should be 4-20 ounces and preferably 8-14 ounces.

In some embodiments, shaft-grounding device 16 comprises a brush housing 112 within which a spring-loaded plunger 114 can slide along a generally linear path 116. Housing 112 can be an electrically conductive tubular body having a longitudinal centerline 118. Plunger 114 includes a copper tube 120 with one end 122 that crimps the copper strands of brush 104 to grounding wire 106. A pin 124 fastens tube 120 to a brass sleeve 126 to complete the assembly of plunger 114. Another pin 128 fixed to housing 112 protrudes into a slot 130 in sleeve 126 to provide an anti-rotation element that not only restricts the rotation of plunger 114 (inhibits shaft 18 from spinning brush 104) but also limits the axial extension of plunger 114 relative to housing 112. A nut 132 with an internal shoulder 134 screws onto to housing 112 to clamp an electrically conductive plug 136 between shoulder 134 and one end 138 of housing 112. An electrical terminal 140 connects grounding wire 106 to plug 136.

When brush housing 112 is screwed into a threaded hole 142 in endplate 144 of motor housing 28, spring 108 is compressed a certain degree between sleeve 126 and plug 136. The characteristics of spring 108 and the amount it is compressed determines the force that brush 104 exerts against bolt head 18′ or against some other axial surface of shaft 18. To minimize rubbing between shaft 18 and brush 104, a rotational axis 146 of shaft 18, the longitudinal centerline 118 of housing 112, and the linear path 116 along which brush 104 and plunger 114 can move are generally collinear with each other.

When properly installed, shaft-grounding device 16 completes an electrical path between shaft 18 and motor housing 28. More specifically, induced common mode current in shaft 18 can travel in series through shaft 18, wire brush 104, grounding wire 106, terminal 140, plug 136, brush housing 112, endplate 144, bearing bracket 66, shell 72 of motor housing 28, and ground 96. Other electrical paths are also possible such as, for example, series flow through shaft 18, brush 104, tube 120, sleeve 126, brush housing 112, endplate 144, bearing bracket 66, compressor shell 72, and ground 96.

During normal operation, an O-ring 148 between housing 112 and endplate 144 plus another O-ring 150 between plug 136 and housing 112 helps maintain the hermetic integrity of compressor system 10. With certain refrigerants and temperature conditions, however, it may still be possible to inspect, replace, repair or otherwise service shaft-grounding device 16 without losing a significant amount of refrigerant charge or introducing non-condensable air into compressor system 10.

In some embodiments, for example, the refrigerant in system 10 is R123, which begins boiling at atmospheric pressure (14.7 psig) when its temperature is about 81.7° F. So, if the temperature of the refrigerant in motor housing ooo is adjusted to about 81.7° F., or slightly less, the refrigerant pressure within motor housing 28 will be about the same as the surrounding atmospheric pressure. Under these conditions, shaft-grounding device 16 can be momentarily unscrewed from within hole 142 and inspected without an excessive exchange of gas between system 10 and the surround atmosphere 32, provided opening 142 is not too large. Preferably, opening 142 should have a cross-sectional area that is less than 4 in².

The refrigerant pressure within system 10 can be adjusted to atmospheric pressure by adjusting the temperature of the refrigerant, which can be done in various ways. The water flow rate through evaporator 40, for instance, could be adjusted while compressor system 10 is de-energized. It is also conceivable to heat or cool the refrigerant by adjusting the temperature and flow rate of the water flowing through condenser 40.

FIG. 5 illustrates the steps of adjusting the temperature of the refrigerant until the refrigerant in motor housing 28 is at a pressure substantially equal to that of the surrounding atmosphere 32, and removing shaft-grounding device 16 from within motor housing 28 while the pressure of the refrigerant is substantially equal to that of the surrounding atmosphere, thereby providing an opportunity to inspect shaft-grounding device 16 without having to evacuate the refrigerant from within motor housing 28. This is possible because compressor system 10 has only one duplex rolling element bearing 22 that needs protection from induced common mode voltage. The shaft's other bearing 58, which is installed at a less accessible location deep within compressor system 10, is a journal bearing which is much more tolerant of induced common mode voltage, thus bearing 58 does not need the same protection as bearing 22.

Although the invention is described with respect to a preferred embodiment, modifications thereto will be apparent to those of ordinary skill in the art. In some embodiments, for instance, motor housing 28 can be considered to comprise cylindrical shell 72, bearing bracket 66 and endplate 144. And in some embodiments, shaft 18 can be considered to include bolt head 18′ and/or other items extending from or attached to shaft 18. Although refrigerant circuit 38 is shown comprising a water-cooled condenser and an evaporator providing chilled water, condenser 40 could be air-cooled and the cooling effect of evaporator 4 could be used for absorbing heat from something other than water. Therefore, the scope of the invention is to be determined by reference to the following claims. 

1. A compressor system powered by an AC voltage supply for compressing a refrigerant, comprising: a compressor housing defining a suction inlet and a discharge outlet; a motor housing extending from the compressor housing; a bearing bracket extending from the motor housing; a bearing having an outer periphery supported by the bearing bracket; a shaft supported by the bearing and being rotatable about a rotational axis, the shaft includes an outboard end and an inboard end, the bearing is closer to the outboard end than to the inboard end; a compressor element driven by the shaft and being rotatable relative to the compressor housing to force the refrigerant from the suction inlet to the discharge outlet, the compressor element is closer to the inboard end than to the outboard end; and a minimally conductive coating disposed on the bearing bracket, the coating is between the bearing bracket and the bearing to provide electrical resistance therebetween, the coating is harder than the bearing bracket and the outer periphery of the bearing.
 2. The compressor system of claim 1 wherein the minimally conductive coating is a ceramic coating including titanium dioxide and aluminum oxide.
 3. The compressor system of claim 1, further comprising: a seal ring attached to the bearing bracket, the seal ring includes a labyrinth seal in proximity to the shaft; and a clamp ring attached to the bearing bracket, wherein the bearing is axially clamped between the seal ring and the clamp ring, and the ceramic coating is further disposed on the seal ring and the clamp ring.
 4. The compressor system of claim 3, further comprising: a rotor carried by the shaft, the rotor is closer to the outboard end than to the inboard end; a stator disposed within the motor housing and encircling the rotor; an inverter electrically coupled to the stator for driving the compressor element at various speeds, the inverter comprises a converter section for converting the AC voltage supply to DC voltage, and an inverter section electrically coupled to the converter section for converting the DC voltage to a variable frequency output that goes to the stator, at least one of the converter section and the inverter section includes a plurality of insulate gate bipolar transistors that can induce a common mode current in the shaft, wherein the common mode current at times exceeds one megahertz.
 5. The compressor system of claim 4, further comprising: a shaft-grounding device comprising a wire brush, a brush housing, and a spring, wherein: a) the brush housing is coupled to the motor housing, b) the wire brush is movable along a longitudinal centerline of the brush housing, c) the spring urges the wire brush toward the outboard end of the shaft to create electrical continuity between the wire brush and the outboard end of the shaft, d) the wire brush is electrically coupled to the brush housing, thereby establishing electrical continuity between the compressor housing and the outboard end of the shaft while the ceramic coating provides electrical resistance directly between the bearing and the bearing bracket.
 6. The compressor system of claim 2, further comprising: a shaft-grounding device comprising a wire brush, a brush housing, and a spring, wherein: e) the brush housing is coupled to the motor housing, f) the wire brush is movable along a longitudinal centerline of the brush housing, g) the spring urges the wire brush toward the outboard end of the shaft to create electrical continuity between the wire brush and the outboard end of the shaft, h) the wire brush is electrically coupled to the brush housing, thereby establishing electrical continuity between the compressor housing and the outboard end of the shaft while the ceramic coating provides electrical resistance directly between the bearing and the bearing bracket.
 7. The compressor system of claim 6, wherein the longitudinal centerline and the rotational axis are substantially collinear.
 8. The compressor system of claim 6, wherein the spring urges the wire brush to exert 4 to 20 ounces of force against the outboard end of the shaft.
 9. The compressor system of claim 6, wherein the shaft-grounding device further comprises an anti-rotation element disposed within the brush housing, wherein the anti-rotation element limits relative rotational movement between the wire brush and the brush housing.
 10. The compressor system of claim 6, wherein the shaft-grounding device further comprises a stranded grounding wire disposed within the brush housing for conveying the common mode current from the wire brush to the brush housing.
 11. A compressor system powered by an AC voltage supply for compressing a refrigerant, comprising: a compressor housing defining a suction inlet and a discharge outlet; a motor housing attached to the compressor housing; a bearing bracket attached to the motor housing; a bearing supported by the bearing bracket; a shaft supported by the bearing and being rotatable about a rotational axis, the shaft includes an outboard end and an inboard end, the bearing is closer to the outboard end than to the inboard end; a compressor element driven by the shaft and being rotatable to force the refrigerant from the suction inlet to the discharge outlet, the compressor element is closer to the inboard end than to the outboard end; an endplate spaced apart from the shaft, spaced apart from the bearing, and electrically coupled to the compressor housing; a ceramic coating disposed on the bearing bracket, the ceramic coating is between the bearing bracket and the bearing; and a shaft-grounding device comprising a wire brush, a brush housing, and a spring, wherein: a) the brush housing is attached to the endplate, b) the wire brush is movable along a longitudinal centerline of the brush housing, c) the spring urges the wire brush toward the outboard end of the shaft to create electrical continuity between the wire brush and the outboard end of the shaft, d) the wire brush is electrically coupled to the brush housing, and e) the brush housing is electrically coupled to the endplate, thereby establishing electrical continuity between the compressor housing and the outboard end of the shaft while the ceramic coating provides electrical resistance directly between the bearing and the bearing bracket.
 12. The compressor system of claim 11, further comprising: a rotor carried by the shaft, the rotor is closer to the outboard end than to the inboard end; a stator disposed within the motor housing and encircling the rotor; an inverter electrically coupled to the stator for driving the compressor element at various speeds, the inverter comprises a converter section for converting the AC voltage supply to DC voltage, and an inverter section electrically coupled to the converter section for converting the DC voltage to a variable frequency output that goes to the stator, both the converter section and the inverter section include a plurality of insulate gate bipolar transistors that can induce a common mode current in the shaft, wherein the common mode current exceeds one megahertz.
 13. The compressor system of claim 12, wherein the longitudinal centerline and the rotational axis are substantially collinear.
 14. The compressor system of claim 12, wherein the spring urges the wire brush to exert 4 to 20 ounces of force against the outboard end of the shaft.
 15. The compressor system of claim 12, wherein the shaft-grounding device further comprises an anti-rotation element disposed within the brush housing, wherein the anti-rotation element limits relative rotational movement between the wire brush and the brush housing.
 16. The compressor system of claim 12, wherein the shaft-grounding device further comprises a grounding wire disposed within the brush housing for conveying the common mode current from the wire brush to the brush housing.
 17. The compressor system of claim 12, further comprising: a seal ring attached to the bearing bracket, the seal ring includes a labyrinth seal in proximity to the shaft; and a clamp ring attached to the bearing bracket, wherein the bearing is axially clamped between the seal ring and the clamp ring, and the ceramic coating is further disposed on the seal ring and the clamp ring.
 18. The compressor system of claim 12, wherein the ceramic coating is harder than the bearing bracket.
 19. The compressor system of claim 12, wherein the ceramic coating is harder than an outer periphery of the bearing.
 20. The compressor system of claim 12, wherein the ceramic coating includes titanium dioxide and aluminum oxide.
 21. A compressor system powered by an AC voltage supply for compressing a refrigerant, comprising: a compressor housing defining a suction inlet and a discharge outlet; a motor housing adjacent to the compressor housing; a bearing supported within the motor housing; a shaft supported by the bearing and being rotatable about a rotational axis, the shaft includes an outboard end and an inboard end, the bearing is closer to the outboard end than to the inboard end; a compressor element driven by the shaft and being rotatable to force the refrigerant from the suction inlet to the discharge outlet, the compressor element is closer to the inboard end than to the outboard end; a rotor carried by the shaft, the rotor is closer to the outboard end than to the inboard end; a stator disposed within the motor housing and encircling the rotor; an inverter electrically coupled to the stator for driving the compressor element at various speeds, the inverter includes a plurality of insulate gate bipolar transistors that can induce a common mode current in the shaft, wherein the common mode current can exceed one megahertz; and a shaft-grounding device comprising a wire brush, a brush housing, a spring, and a grounding wire, wherein: a) the brush housing is electrically coupled to the motor housing, b) the grounding wire electrically couples the wire brush to the brush housing; c) the wire brush is movable along a substantially linear path, and d) the spring is contained within the brush housing and urges the wire brush toward the outboard end of the shaft to create electrical continuity between the wire brush and the outboard end of the shaft.
 22. The compressor system of claim 21, wherein the substantially linear path and the rotational axis are substantially collinear.
 23. The compressor system of claim 21, wherein the spring urges the wire brush to exert 4 to 20 ounces of force against the outboard end of the shaft.
 24. The compressor system of claim 21, wherein the shaft-grounding device further comprises an anti-rotation element disposed within the brush housing, wherein the anti-rotation element limits relative rotational movement between the wire brush and the brush housing.
 25. The compressor system of claim 21 further including a bearing bracket supporting the bearing and a non-conductive coating on the bearing bracket.
 26. A compressor system, comprising: a motor housing having an interior containing a refrigerant and an exterior exposed to a surrounding atmosphere; a compressor housing hermetically sealed to the motor housing; an endplate extending from the motor housing; a refrigerant disposed within the compressor housing and the motor housing; a rolling element bearing inside the motor housing; a journal bearing inside at least one of the motor housing and the compressor housing; a shaft having an inboard end, an outboard end, and an intermediate section therebetween, wherein the rolling element bearing supports the outboard end, and the journal bearing supports the intermediate section; a compressor element mounted to the inboard end of the shaft and being rotatable for compressing the refrigerant; a shaft-grounding device extending into the opening of the endplate such that the shaft-grounding device is in electrical contact with the outboard end of the shaft and is exposed to the refrigerant and the surrounding atmosphere.
 27. The compressor system of claim 26, further comprising a minimally conductive material interposed between the rolling element bearing and the motor housing.
 28. The compressor system of claim 27 wherein the minimally conductive material is a ceramic coating, a metal oxide, or a silicon oxide.
 29. The compressor system of claim 28, further comprising a rotor carried by the shaft between the rolling element bearing and the journal bearing, a stator encircling the rotor, an inverter powering the stator to rotate the rotor, the shaft, and the compressor element at various speeds, wherein the inverter includes a plurality of insulate gate bipolar transistors.
 30. The compressor system of claim 26, wherein the endplate defines an opening whose cross-sectional area is less than 4 in² and wherein the opening of the endplate is a threaded hole that threadingly engages the shaft-grounding device.
 31. A method of servicing a hermetically sealed compressor system that includes a motor housing exposed to a surrounding atmosphere, a shaft rotatable within the motor housing, a refrigerant disposed within the motor housing, and a shaft-grounding device that when properly installed is exposed to the refrigerant and the surrounding atmosphere and completes an electrical path between the shaft and the motor housing, the method comprising: adjusting the temperature of the refrigerant until the refrigerant in the motor housing is at a pressure substantially equal to that of the surrounding atmosphere; and removing the shaft-grounding device from within the motor housing while the pressure of the refrigerant is substantially equal to that of the surrounding atmosphere, thereby providing an opportunity to inspect the shaft-grounding device without having to evacuate the refrigerant from within the motor housing.
 32. The method of claim 31, wherein the shaft-grounding device includes a spring, and the method of servicing a hermetically sealed compressor further comprises at least partially relaxing the spring upon removing the shaft-grounding device. 